Monotonic transactions in a multi-master database with loosely coupled nodes

ABSTRACT

One embodiment provides for monotonic transactions in a multi-master database with loosely coupled nodes including performing, by a processor, a write transaction protocol including: a client device issuing write transactions at any of the nodes, recording a write transaction locally at a node that issues a write transaction and asynchronously replicating the write transaction to at least one other node, and waiting for reception of an acknowledgment from at least a quorum of the nodes before returning a response to the client device. The quorum is any set of the nodes that includes a special node and at least one other node.

BACKGROUND

Online transaction processing (OLTP) applications are used for databasesthat provide ACID (high atomicity, consistency, isolation anddurability) consistency properties, but are increasingly working inloosely coupled “AP” (availability, partition tolerance) settings whereconsistency guarantees are not strong. In particular, when a transactioninserts rows and commits, the application expects one of three returns:a) transaction success, b) transaction rolls back (e.g., constraintviolation), c) transaction status unknown (usually due to a driver orconnection error, and an application can issue queries to check thestatus). But in any case, after a commit, in conventional ACID databasemanagement systems (DBMSs), subsequent reads see the same answers a(i.e., monotonic behavior). If one query sees the inserted rows, thensubsequent queries will also see those inserted rows, consistently (forreturn c),an app has to issue a read to find out status).

In an AP environment (e.g., with many NoSQL (non-structured querylanguage) DBMSs), there is a fourth return, referring to a problematicreturn state: d) transaction status may not be settled yet. This happenswhen the node (where a node is a client device, a server, or peerdevice) running the transaction has sent the changes/inserts (as a logmessage) to other replicas, but has not heard acknowledgements from them(indicating that they received or accepted the changes). In this state,subsequent queries see unpredictable behavior. A first query may notfind those rows—because they have not replicated yet to sufficientnumber of replicas (according to a quorum policy). But a later query mayfind those rows.

SUMMARY

Embodiments relate to quorum processing for replication in amulti-master database with loosely coupled nodes. One embodimentprovides a method for monotonic transactions in a multi-master databasewith loosely coupled nodes including performing, by a processor, a writetransaction protocol including: a client device issuing writetransactions at any of the nodes, recording a write transaction locallyat a node that issues a write transaction and asynchronously replicatingthe write transaction to at least one other node, and waiting forreception of an acknowledgment from at least a quorum of the nodesbefore returning a response to the client device. The quorum is any setof the nodes that includes a special node and at least one other node.

These and other features, aspects and advantages of the presentinvention will become understood with reference to the followingdescription, appended claims and accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a cloud computing environment, according to anembodiment;

FIG. 2 depicts a set of abstraction model layers, according to anembodiment;

FIG. 3 is a network architecture for efficient representation, accessand modification of variable length data objects, according to anembodiment;

FIG. 4 shows a representative hardware environment that may beassociated with the servers and/or clients of FIG. 1, according to anembodiment;

FIG. 5 is a block diagram illustrating system for quorum processing forreplication in a multi-master database with loosely coupled nodes,according to one embodiment; and

FIG. 6 illustrates a block diagram for a process for monotonictransactions in a multi-master database with loosely coupled nodes,according to one embodiment.

DETAILED DESCRIPTION

The descriptions of the various embodiments have been presented forpurposes of illustration, but are not intended to be exhaustive orlimited to the embodiments disclosed. Many modifications and variationswill be apparent to those of ordinary skill in the art without departingfrom the scope and spirit of the described embodiments. The terminologyused herein was chosen to best explain the principles of theembodiments, the practical application or technical improvement overtechnologies found in the marketplace, or to enable others of ordinaryskill in the art to understand the embodiments disclosed herein.

It is understood in advance that although this disclosure includes adetailed description of cloud computing, implementation of the teachingsrecited herein are not limited to a cloud computing environment. Rather,embodiments of the present invention are capable of being implemented inconjunction with any other type of computing environment now known orlater developed.

One or more embodiments provide for quorum processing for replication ina multi-master database with loosely coupled nodes. One embodimentprovides a method for monotonic transactions in a multi-master databasewith loosely coupled nodes including performing, by a processor, a writetransaction protocol including: a client device issuing writetransactions at any of the nodes, recording a write transaction locallyat a node that issues a write transaction and asynchronously replicatingthe write transaction to at least one other node, and waiting forreception of an acknowledgment from at least a quorum of the nodesbefore returning a response to the client device. The quorum is any setof the nodes that includes a special node and at least one other node.

Cloud computing is a model of service delivery for enabling convenient,on-demand network access to a shared pool of configurable computingresources (e.g., networks, network bandwidth, servers, processing,memory, storage, applications, virtual machines (VMs), and services)that can be rapidly provisioned and released with minimal managementeffort or interaction with a provider of the service. This cloud modelmay include at least five characteristics, at least three servicemodels, and at least four deployment models.

Characteristics are as follows:

On-demand self-service: a cloud consumer can unilaterally provisioncomputing capabilities, such as server time and network storage, asneeded and automatically, without requiring human interaction with theservice's provider.

Broad network access: capabilities are available over a network andaccessed through standard mechanisms that promote use by heterogeneous,thin or thick client platforms (e.g., mobile phones, laptops, and PDAs).

Resource pooling: the provider's computing resources are pooled to servemultiple consumers using a multi-tenant model, with different physicaland virtual resources dynamically assigned and reassigned according todemand. There is a sense of location independence in that the consumergenerally has no control or knowledge over the exact location of theprovided resources but may be able to specify location at a higher levelof abstraction (e.g., country, state, or data center).

Rapid elasticity: capabilities can be rapidly and elasticallyprovisioned and, in some cases, automatically, to quickly scale out andrapidly released to quickly scale in. To the consumer, the capabilitiesavailable for provisioning often appear to be unlimited and can bepurchased in any quantity at any time.

Measured service: cloud systems automatically control and optimizeresource use by leveraging a metering capability at some level ofabstraction appropriate to the type of service (e.g., storage,processing, bandwidth, and active consumer accounts). Resource usage canbe monitored, controlled, and reported, thereby providing transparencyfor both the provider and consumer of the utilized service.

Service Models are as follows:

Software as a Service (SaaS): the capability provided to the consumer isthe ability to use the provider's applications running on a cloudinfrastructure. The applications are accessible from various clientdevices through a thin client interface, such as a web browser (e.g.,web-based email). The consumer does not manage or control the underlyingcloud infrastructure including network, servers, operating systems,storage, or even individual application capabilities, with the possibleexception of limited consumer-specific application configurationsettings.

Platform as a Service (PaaS): the capability provided to the consumer isthe ability to deploy onto the cloud infrastructure consumer-created oracquired applications created using programming languages and toolssupported by the provider. The consumer does not manage or control theunderlying cloud infrastructure including networks, servers, operatingsystems, or storage, but has control over the deployed applications andpossibly application-hosting environment configurations.

Infrastructure as a Service (IaaS): the capability provided to theconsumer is the ability to provision processing, storage, networks, andother fundamental computing resources where the consumer is able todeploy and run arbitrary software, which can include operating systemsand applications. The consumer does not manage or control the underlyingcloud infrastructure but has control over operating systems, storage,deployed applications, and possibly limited control of select networkingcomponents (e.g., host firewalls).

Deployment Models are as follows:

Private cloud: the cloud infrastructure is operated solely for anorganization. It may be managed by the organization or a third party andmay exist on-premises or off-premises.

Community cloud: the cloud infrastructure is shared by severalorganizations and supports a specific community that has shared concerns(e.g., mission, security requirements, policy, and complianceconsiderations). It may be managed by the organizations or a third partyand may exist on-premises or off-premises.

Public cloud: the cloud infrastructure is made available to the generalpublic or a large industry group and is owned by an organization sellingcloud services.

Hybrid cloud: the cloud infrastructure is a composition of two or moreclouds (private, community, or public) that remain unique entities butare bound together by standardized or proprietary technology thatenables data and application portability (e.g., cloud bursting for loadbalancing between clouds).

A cloud computing environment is a service oriented with a focus onstatelessness, low coupling, modularity, and semantic interoperability.At the heart of cloud computing is an infrastructure comprising anetwork of interconnected nodes.

Referring now to FIG. 1, an illustrative cloud computing environment 50is depicted. As shown, cloud computing environment 50 comprises one ormore cloud computing nodes 10 with which local computing devices used bycloud consumers, such as, for example, personal digital assistant (PDA)or cellular telephone 54A, desktop computer 54B, laptop computer 54C,and/or automobile computer system 54N may communicate. Nodes 10 maycommunicate with one another. They may be grouped (not shown) physicallyor virtually, in one or more networks, such as private, community,public, or hybrid clouds as described hereinabove, or a combinationthereof. This allows the cloud computing environment 50 to offerinfrastructure, platforms, and/or software as services for which a cloudconsumer does not need to maintain resources on a local computingdevice. It is understood that the types of computing devices 54A-N shownin FIG. 2 are intended to be illustrative only and that computing nodes10 and cloud computing environment 50 can communicate with any type ofcomputerized device over any type of network and/or network addressableconnection (e.g., using a web browser).

Referring now to FIG. 2, a set of functional abstraction layers providedby the cloud computing environment 50 (FIG. 1) is shown. It should beunderstood in advance that the components, layers, and functions shownin FIG. 2 are intended to be illustrative only and embodiments of theinvention are not limited thereto. As depicted, the following layers andcorresponding functions are provided:

Hardware and software layer 60 includes hardware and softwarecomponents. Examples of hardware components include: mainframes 61; RISC(Reduced Instruction Set Computer) architecture based servers 62;servers 63; blade servers 64; storage devices 65; and networks andnetworking components 66. In some embodiments, software componentsinclude network application server software 67 and database software 68.

Virtualization layer 70 provides an abstraction layer from which thefollowing examples of virtual entities may be provided: virtual servers71; virtual storage 72; virtual networks 73, including virtual privatenetworks; virtual applications and operating systems 74; and virtualclients 75.

In one example, a management layer 80 may provide the functionsdescribed below. Resource provisioning 81 provides dynamic procurementof computing resources and other resources that are utilized to performtasks within the cloud computing environment. Metering and pricing 82provide cost tracking as resources are utilized within the cloudcomputing environment and billing or invoicing for consumption of theseresources. In one example, these resources may comprise applicationsoftware licenses. Security provides identity verification for cloudconsumers and tasks as well as protection for data and other resources.User portal 83 provides access to the cloud computing environment forconsumers and system administrators. Service level management 84provides cloud computing resource allocation and management such thatrequired service levels are met. Service Level Agreement (SLA) planningand fulfillment 85 provide pre-arrangement for, and procurement of,cloud computing resources for which a future requirement is anticipatedin accordance with an SLA.

Workloads layer 90 provides examples of functionality for which thecloud computing environment may be utilized. Examples of workloads andfunctions which may be provided from this layer include: mapping andnavigation 91; software development and lifecycle management 92; virtualclassroom education delivery 93; data analytics processing 94;transaction processing 95; and quorum processing for replication in amulti-master database with loosely coupled nodes 96. As mentioned above,all of the foregoing examples described with respect to FIG. 2 areillustrative only, and the invention is not limited to these examples.

It is understood all functions of one or more embodiments as describedherein may be typically performed by the processing system 300 (FIG. 3)or the autonomous cloud environment 410 (FIG. 4), which can be tangiblyembodied as hardware processors and with modules of program code.However, this need not be the case for non-real-time processing. Rather,for non-real-time processing the functionality recited herein could becarried out/implemented and/or enabled by any of the layers 60, 70, 80and 90 shown in FIG. 2.

It is reiterated that although this disclosure includes a detaileddescription on cloud computing, implementation of the teachings recitedherein are not limited to a cloud computing environment. Rather, theembodiments of the present invention may be implemented with any type ofclustered computing environment now known or later developed.

FIG. 3 illustrates a network architecture 300, in accordance with oneembodiment. As shown in FIG. 3, a plurality of remote networks 302 areprovided, including a first remote network 304 and a second remotenetwork 306. A gateway 301 may be coupled between the remote networks302 and a proximate network 308. In the context of the present networkarchitecture 300, the networks 304, 306 may each take any formincluding, but not limited to, a LAN, a WAN, such as the Internet,public switched telephone network (PSTN), internal telephone network,etc.

In use, the gateway 301 serves as an entrance point from the remotenetworks 302 to the proximate network 308. As such, the gateway 301 mayfunction as a router, which is capable of directing a given packet ofdata that arrives at the gateway 301, and a switch, which furnishes theactual path in and out of the gateway 301 for a given packet.

Further included is at least one data server 314 coupled to theproximate network 308, which is accessible from the remote networks 302via the gateway 301. It should be noted that the data server(s) 314 mayinclude any type of computing device/groupware. Coupled to each dataserver 314 is a plurality of user devices 316. Such user devices 316 mayinclude a desktop computer, laptop computer, handheld computer, printer,and/or any other type of logic-containing device. It should be notedthat a user device 311 may also be directly coupled to any of thenetworks in some embodiments.

A peripheral 320 or series of peripherals 320, e.g., facsimile machines,printers, scanners, hard disk drives, networked and/or local storageunits or systems, etc., may be coupled to one or more of the networks304, 306, 308. It should be noted that databases and/or additionalcomponents may be utilized with, or integrated into, any type of networkelement coupled to the networks 304, 306, 308. In the context of thepresent description, a network element may refer to any component of anetwork.

According to some approaches, methods and systems described herein maybe implemented with and/or on virtual systems and/or systems, whichemulate one or more other systems, such as a UNIX system that emulatesan IBM z/OS environment, a UNIX system that virtually hosts a MICROSOFTWINDOWS environment, a MICROSOFT WINDOWS system that emulates an IBMz/OS environment, etc. This virtualization and/or emulation may beimplemented through the use of VMWARE software in some embodiments.

FIG. 4 shows a representative hardware system 400 environment associatedwith a user device 316 and/or server 314 of FIG. 3, in accordance withone embodiment. In one example, a hardware configuration includes aworkstation having a central processing unit 410, such as amicroprocessor, and a number of other units interconnected via a systembus 412. The workstation shown in FIG. 4 may include a Random AccessMemory (RAM) 414, Read Only Memory (ROM) 416, an I/O adapter 418 forconnecting peripheral devices, such as disk storage units 420 to the bus412, a user interface adapter 422 for connecting a keyboard 424, a mouse426, a speaker 428, a microphone 432, and/or other user interfacedevices, such as a touch screen, a digital camera (not shown), etc., tothe bus 412, communication adapter 434 for connecting the workstation toa communication network 435 (e.g., a data processing network) and adisplay adapter 436 for connecting the bus 412 to a display device 438.

In one example, the workstation may have resident thereon an operatingsystem, such as the MICROSOFT WINDOWS Operating System (OS), a MAC OS, aUNIX OS, etc. In one embodiment, the system 400 employs a POSIX° basedfile system. It will be appreciated that other examples may also beimplemented on platforms and operating systems other than thosementioned. Such other examples may include operating systems writtenusing JAVA, XML, C, and/or C++ language, or other programming languages,along with an object oriented programming methodology. Object orientedprogramming (OOP), which has become increasingly used to develop complexapplications, may also be used.

In conventional replicated databases, a data object has copies presentat multiple locations. A vital consistency guarantee that such databasesgive to applications is monotonic reads. A write operation (also knownconventionally as a transaction) modifies the database state atomically,and all subsequent reads (queries) either see the operation'smodification (in which case the operation is said to have succeeded orcommitted), or not see the operation's modification (in which case theoperation is said to have rolled back). In conventional replicateddatabases, this guarantee is achieved through a quorum-based replicacontrol protocol with synchronous, blocking writes. Specifically, eachcopy of a replicated data item is assigned a vote. Each operation thenhas to obtain a read quorum (T_(r)) or a write quorum (T_(w)) to read orwrite a data item, respectively. If a given data item has a total of Tvotes, the quorums have to obey the following rules:

T _(r) +T _(w) >T   1.

T _(w) >T/2   2.

A write operation has to wait until the modifications from that writehave replicated to a write quorum of replicas, and this has to beverified by waiting for acknowledgements. Further, if this waitingprocess fails or times-out, the write has to be rolled back. This istypically achieved with a two-phase commit protocol. Thus, these rulesensure that two write quorums always have a common node, and that anyread quorum contains at least one site with the newest version of thedata item. Therefore, either a write succeeds or rolls-back atomically,and all subsequent reads see a consistent image of the database. In suchconsistent systems, there is also a third return situation possible:write operation status unknown. This can happen for example when theconnection between database and application fails, or the applicationtimes-out when waiting for a response from the database. But still, evenin such situation, the application can query the database for the statusof the write operation—and that query (all subsequent queries), will seeconsistent answers. The disadvantage of this approach is that itrequires a tightly connected system where nodes are available andresponsive. For example, once a two-phase commit protocol has begun, ifthe initiator node becomes unresponsive, the system is unavailable tosubsequent reads.

Distinguishable from consistent systems having three return situations(i.e., success, failure with rollback and unknown), in eventualconsistency systems applications eventually get monotonic reads, butthere is some time after the completion of a write when reads may seeinconsistent answers. When the database state is modified by a writeoperation, the return statuses are success and unknown. In the lastcase, subsequent reads get varying answers. If a read goes to one of thenodes to which the modification has been replicated, the read sees themodification, and a later read (that goes to a different node) might notsee the modifications; then another read sees the modifications. Buteventual consistency systems have high availability. Writes can happenat any node (“multi-master”); asynchronously replicated to other nodes;and the system can tolerate failures of some number of nodes.

Multi-master replication (or peer-to-peer n-way replication) is a methodof database replication which provides for data to be stored by a groupof computers, and updated by any member of the group. Updates made to anindividual master site are propagated to all other master sites.Multi-master replication provides convergence of the data of alldatabase table replicas, and provides global transaction consistency anddata integrity. Conflict resolution is independently handled at each ofthe master nodes. Multi-master replication provides complete replicas ofeach replicated database table at each of the master nodes. All membersare responsive to client data queries. The multi-master replicationsystem is responsible for propagating the data modifications made byeach member to the rest of the group, and resolving any conflicts thatmight arise between concurrent changes made by different members.Multi-master replication provides increased availability and fasterserver response time.

In conventional ACID DBMSs, the four states of atomicity, consistency,isolation and durability provides the following. For atomicity, databasemodification transactions (i.e., atomic transactions) must follow an“all or nothing” rule. If one portion of a transaction fails, the entiretransaction fails. For consistency, only valid data will be written tothe database. If a transaction is executed that violates the database'sconsistency rules, the entire transaction will be rolled back, and thedatabase will be restored to a state consistent with those rules. If atransaction successfully executes, it will take the database from onestate that is consistent with the rules to another state that is alsoconsistent with the rules. For isolation, multiple transactionsoccurring at the same time do not impact each other's execution. For twosimultaneous issued transactions against a database, both transactionsoperate on the database in an isolated manner. The database eitherperforms one entire transaction before executing the other, orvice-versa. This prevents one transaction from reading intermediate dataproduced as a side effect of part of the other transaction that will noteventually be committed to the database. The isolation property does notensure which transaction will execute first, but that they will notinterfere with each other. For durability, any transaction committed tothe database will not be lost. Durability uses backups and transactionlogs that facilitate the restoration of committed transactionsregardless of subsequent failures (e.g., software or hardware failures).

FIG. 5 is a block diagram illustrating a system 500 for quorumprocessing for replication in a multi-master database with looselycoupled nodes, according to one embodiment. In one embodiment, thesystem 500 includes client devices 510 (e.g., mobile devices, smartdevices, computing systems, etc.), a cloud or resource sharingenvironment 520, and servers 530. In one embodiment, the client devicesare provided with cloud services from the servers 530 through the cloudor resource sharing environment 520. In one embodiment, system 500 is amulti-master database system that provides that one node is elected as aspecial node (or leader node) using a consensus protocol. Consensus isthe process of agreeing on one result among a group of participants. Inone embodiment, in system 500, for a write quorum, the special node andat least one non-special (or non-leader) node must be part of any writequorum. For a read quorum, the special node and at least one non-specialnode must be part of any read quorum. Note that both the write quorumand the read quorum may demand additional nodes to be participants,depending on durability requirements. After a commit transaction, theinitiator node waits for that transaction's log to be replicated to awrite-quorum of nodes (and receive an acknowledgement (ack)) beforereturning a SUCCESS result notification. The initiator node nevertimes-out while waiting for acks. An application may time out, in whichcase the return state is transaction status UNKNOWN.

In one embodiment, in system 500 writes (transactions) can go to anynode. In one embodiment, the system 500 adopts a quorum-readquorum-write protocol where reads see only data that is visible on aquorum of nodes and a quorum is one or more sets of nodes. In oneexample embodiment, Quorum is any set with at least a majority of nodes.The system 500 maintains an elected Special-Node, and uses a writeprotocol and write quorum where: writes wait until they are notifiedwith an acknowledgement (that the write was replicated) from at leastthe write quorum of nodes, writes operations never timeout, and any readquorum and any write quorum always contains a special node and at leastone other node.

In one or more embodiments, it should be noted that the special node mayfail at any time and a new special node may have to be elected. This iswhy a special-only quorum does not work. When a new special node iselected, it is guaranteed that any successful transactions (atransaction is successful if either the commit returns SUCCESS or itreturned the state transaction status UNKNOWN, and a subsequent querysaw the modifications of the transaction) have their respective changespropagated to the previous special node plus at least one other node.The other node may be used to recover all successful transactions, evenif the previous special node is now unresponsive.

In one embodiment, in system 500, the write protocol includes that thesystem 500 may: 1) issue a write at any node; 2) records the writelocally (at that node) and asynchronously replicated to others; 3) waitsuntil acknowledgements of receipt has been received from at least aquorum of nodes; and 4) where a client device 510 can timeout during 3),and then find out the status of write by issuing reads.

In one embodiment, in system 500, the read protocol includes that thesystem 500 may: 1) issue a read at any node; 2) a read only returns rowsthat are known to be replicated at least a quorum of nodes; and 3) ifthe read at 2) times out because the special node is not responsive, thesystem 500 elects a new special node via a consensus protocol, andcontinues to 2).

In one example embodiment, the system 500: 1) issues a write at node A(which is elected as a special node), where system 500 has other nodesB, C, D and E; 2) a client device 510 times out and issues a read at anynode, for example node C. In this example, in system 500 a read onlyreturns rows that are visible at A and one other node. If node C hasreceived the write in 1), then the read sees the write. Otherwise, nodeC waits to receive notification from node A, B, D, and/or node E. Ifnode C gets notified that node A and at least one of nodes B, C, D, andE have received the write, then the read sees the write. If the processtimes out waiting for node A and none of nodes B, C, D and E havereceived the write yet, a new special node is elected. The originalwrite is lost. Otherwise, the write is always visible to this read andsubsequent reads.

In one embodiment, a weighting scheme may be implemented by system 500as follows. Assume there are three nodes. The special node may have aweight of 0.9. The other two nodes have a weight of 0.3 each. In oneembodiment, a threshold for a quorum is that the sum of weights must be≥1.2, which is the special node plus one other node. It should be notedthat other weighting schemes may also be used accordingly.

One or more embodiments provide the following benefits: transactions areable to commit even in a loosely coupled environment where the networkmay get partitioned or nodes may fail; transactions can be submitted toany node, not just a leader node; even when the return status of atransaction is unknown, the application still has monotonic reads. Theread quorum involves a special node, so it can never be the case thatone read misses the inserted rows and a subsequent read finds them.

FIG. 6 illustrates a block diagram for a process 600 for monotonictransactions in a multi-master database with loosely coupled nodes,according to one embodiment. In block 610, process 600 performs, by aprocessor (e.g., a processor in cloud computing environment 50, FIG. 1,system 300, FIG. 3, system 400, FIG. 4, system 500, FIG. 5), a writetransaction protocol. In block 620, process 600 continues to perform thewrite transaction protocol that includes: a client device issuing writetransactions at any of the nodes (e.g., a client device 510, a server530, or any other peer device in the cloud computing environment 50,system 300, system 400, or system 500), recording (the modifications of)a write transaction locally at a node that issues a write transactionand asynchronously replicating the write transaction to at least oneother node, and waiting for reception of an acknowledgment from at leasta quorum of the nodes before returning a response to the client device.In one embodiment, the quorum is any set of the nodes that includes aspecial node and at least one other node (and optionally other nodes).

Process 600 may further include performing, by the processor, a readtransaction protocol that includes: issuing read transactions at any ofthe nodes, attempting to return rows that are known to be replicated byat least a quorum of the nodes, and for timing out of the attempting toreturn rows known to be replicated by at least the quorum of nodes basedon the special-node being non-responsive: selecting a new special-node,and repeating attempting to return rows that are known to be replicatedby at least the quorum of the nodes.

In one embodiment, process 600 may further upon determining a clientdevice timed out during waiting for receiving an acknowledgment from themulti-master database, the client device obtains status of the writetransaction by issuing at least one read transaction. Then the clientdevice waits for a response to this read transaction; and upon this waitexceeding a timeout threshold, the client device repeats to retry theread transaction. In one embodiment, in process 600 selection of aspecial node includes an election.

In one embodiment, process 600 includes that the special node is any ofthe nodes, and the election is performed by the processor using aconsensus protocol. In one embodiment, in process 600 upon determiningtimeout of the at least one read transaction, the client device repeatsthe at least one read transaction.

In one embodiment, for process 600 selecting of the new special-node isperformed by the processor upon a transaction failure ornon-responsiveness of the special node.

In one embodiment, process 600 may include that the quorum furtherincludes a number of other nodes selected based on a durabilityrequirement. Process 600 may further include assigning weights to thenodes including the special node. For process 600, a weight assigned tothe special-node exceeds weights assigned to remaining nodes, and quorumvoting may be based on a sum of voting nodes weights being equal to orgreater than a quorum threshold.

Any combination of one or more computer readable medium(s) may beutilized. The computer readable medium may be a computer readable signalmedium or a computer readable storage medium. A computer readablestorage medium may be, for example, but not limited to, an electronic,magnetic, optical, electromagnetic, infrared, or semiconductor system,apparatus, or device, or any suitable combination of the foregoing. Morespecific examples (a non-exhaustive list) of the computer readablestorage medium would include the following: an electrical connectionhaving one or more wires, a portable computer diskette, a hard disk, arandom access memory (RAM), a read-only memory (ROM), an erasableprogrammable read-only memory (EPROM or Flash memory), an optical fiber,a portable compact disc read-only memory (CD-ROM), an optical storagedevice, a magnetic storage device, or any suitable combination of theforegoing. In the context of this document, a computer readable storagemedium may be any tangible medium that can contain, or store a programfor use by or in connection with an instruction execution system,apparatus, or device.

A computer readable signal medium may include a propagated data signalwith computer readable program code embodied therein, for example, inbaseband or as part of a carrier wave. Such a propagated signal may takeany of a variety of forms, including, but not limited to,electro-magnetic, optical, or any suitable combination thereof. Acomputer readable signal medium may be any computer readable medium thatis not a computer readable storage medium and that can communicate,propagate, or transport a program for use by or in connection with aninstruction execution system, apparatus, or device.

Program code embodied on a computer readable medium may be transmittedusing any appropriate medium, including but not limited to wireless,wireline, optical fiber cable, RF, etc., or any suitable combination ofthe foregoing.

Computer program code for carrying out operations for aspects of thepresent invention may be written in any combination of one or moreprogramming languages, including an object oriented programming languagesuch as Java, Smalltalk, C++ or the like and conventional proceduralprogramming languages, such as the “C” programming language or similarprogramming languages. The program code may execute entirely on theuser's computer, partly on the user's computer, as a stand-alonesoftware package, partly on the user's computer and partly on a remotecomputer or entirely on the remote computer or server. In the latterscenario, the remote computer may be connected to the user's computerthrough any type of network, including a local area network (LAN) or awide area network (WAN), or the connection may be made to an externalcomputer (for example, through the Internet using an Internet ServiceProvider).

Aspects of the present invention are described below with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems) and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer program instructions. These computer program instructions maybe provided to a processor of a general purpose computer, specialpurpose computer, or other programmable data processing apparatus toproduce a machine, such that the instructions, which execute via theprocessor of the computer or other programmable data processingapparatus, create means for implementing the functions/acts specified inthe flowchart and/or block diagram block or blocks.

These computer program instructions may also be stored in a computerreadable medium that can direct a computer, other programmable dataprocessing apparatus, or other devices to function in a particularmanner, such that the instructions stored in the computer readablemedium produce an article of manufacture including instructions whichimplement the function/act specified in the flowchart and/or blockdiagram block or blocks.

The computer program instructions may also be loaded onto a computer,other programmable data processing apparatus, or other devices to causea series of operational steps to be performed on the computer, otherprogrammable apparatus or other devices to produce a computerimplemented process such that the instructions which execute on thecomputer or other programmable apparatus provide processes forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods, and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof instructions, which comprises one or more executable instructions forimplementing the specified logical function(s). In some alternativeimplementations, the functions noted in the block may occur out of theorder noted in the figures. For example, two blocks shown in successionmay, in fact, be executed substantially concurrently, or the blocks maysometimes be executed in the reverse order, depending upon thefunctionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts or carry out combinations of special purpose hardwareand computer instructions.

References in the claims to an element in the singular is not intendedto mean “one and only” unless explicitly so stated, but rather “one ormore.” All structural and functional equivalents to the elements of theabove-described exemplary embodiment that are currently known or latercome to be known to those of ordinary skill in the art are intended tobe encompassed by the present claims. No claim element herein is to beconstrued under the provisions of 35 U.S.C. section 112, sixthparagraph, unless the element is expressly recited using the phrase“means for” or “step for.”

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription of the present invention has been presented for purposes ofillustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention. Theembodiment was chosen and described in order to best explain theprinciples of the invention and the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated.

What is claimed is:
 1. A method for monotonic transactions in amulti-master database with loosely coupled nodes comprising: performing,by a processor, a write transaction protocol including: a client deviceissuing write transactions at any of the nodes; recording a writetransaction locally at a node that issues a write transaction andasynchronously replicating the write transaction to at least one othernode; and waiting for reception of an acknowledgment from at least aquorum of the nodes before returning a response to the client device,wherein the quorum is any set of the nodes that includes a special nodeand at least one other node.
 2. The method of claim 1, furthercomprising: performing, by the processor, a read transaction protocolincluding: issuing read transactions at any of the nodes; attempting toreturn rows that are known to be replicated by at least a quorum of thenodes; and for timing-out of the attempting to return rows known to bereplicated by at least the quorum of nodes based on the special-nodebeing non-responsive: selecting a new special-node; and repeatingattempting to return rows that are known to be replicated by at leastthe quorum of the nodes.
 3. The method of claim 1, wherein upondetermining a client device timed out during waiting for receiving anacknowledgment from the multi-master database, the client device obtainsstatus of the write transaction by issuing at least one readtransaction.
 4. The method of claim 2, wherein selection of a specialnode comprises an election.
 5. The method of claim 2, wherein thespecial node is any of the nodes, and the election is performed by theprocessor using a consensus protocol.
 6. The method of claim 3, furthercomprising: upon determining timeout of the at least one readtransaction, repeating, by the client device, the at least one readtransaction.
 7. The method of claim 2, wherein the selecting of the newspecial-node is performed by the processor upon a transaction failure ornon-responsiveness of the special node.
 8. The method of claim 5,wherein the quorum further includes a number of other nodes selectedbased on a durability requirement.
 9. The method of claim 2, furthercomprising: assigning weights to the nodes including the special node,wherein a weight assigned to the special-node exceeds weights assignedto remaining nodes, and quorum voting is based on a sum of voting nodesweights being equal to or greater than a quorum threshold.
 10. Acomputer program product for monotonic transactions in a multi-masterdatabase with loosely coupled nodes, the computer program productcomprising a computer readable storage medium having programinstructions embodied therewith, the program instructions executable bya processor to cause the processor to: perform, by the processor, awrite transaction protocol including: a client device issuing writetransactions at any of the nodes; recording a write transaction locallyat a node that issues a write transaction and asynchronously replicatingthe write transaction to at least one other node; and waiting forreception of an acknowledgment from at least the quorum of the nodesbefore returning a response to the client device, wherein the quorum isany set of the nodes that includes a special node and at least one othernode.
 11. The computer program product of claim 9, further comprisingprogram instructions executable by the processor to cause the processorto: perform, by the processor, a read transaction protocol includingfurther program instructions executable by the processor to cause theprocessor to: issue read transactions at any of the nodes; attempt toreturn rows that are known to be replicated by at least the quorum ofthe nodes; and for timing out of the attempting to return rows known tobe replicated by at least the quorum of nodes based on the special-nodebeing non-responsive: select a new special-node; and repeat attemptingto return rows that are known to be replicated by at least the quorum ofthe nodes.
 12. The computer program product of claim 10, wherein upondetermining a client device timed out during waiting for receiving anacknowledgment the multi-master database, the client device obtainsstatus of the write transaction by issuing at least one readtransaction.
 13. The computer program product of claim 12, whereinselection of a special node comprises an election.
 14. The computerprogram product of claim 13, wherein the special node is any of thenodes, and the election is performed by the processor using a consensusprotocol.
 15. The computer program product of claim 11, wherein theselecting of the new special-node is performed by the processor upon atransaction failure or non-responsiveness of the special node.
 16. Thecomputer program product of claim 11, further comprising: upondetermining timeout of the at least one read transaction, repeating, bythe client device, the at least one read transaction.
 17. The computerprogram product of claim 2, wherein selecting of the new special-node isperformed by the processor upon a transaction failure ornon-responsiveness of the special node.
 18. The computer program productof claim 10, wherein the quorum further includes a number of other nodesselected based on a durability requirement.
 19. The computer programproduct of claim 11, further comprising program instructions executableby the processor to cause the processor to: assign, by the processor,weights to the nodes including the special node, wherein a weightassigned to the special-node exceeds weights assigned to remainingnodes, and quorum voting is based on a sum of voting nodes weights beingequal to or greater than a quorum threshold.
 20. An apparatuscomprising: a memory configured to store instructions; and a processorconfigured to execute the instructions to: perform a write transactionprotocol including: a client device issuing write transactions at any ofthe nodes; recording a write transaction locally at a node that issues awrite transaction and asynchronously replicating the write transactionto at least one other node; and waiting for reception of anacknowledgment from at least the quorum of the nodes before returning aresponse to the client device, wherein the quorum is any set of thenodes that includes a special node and at least one other node.
 21. Theapparatus of claim 17, wherein the processor is further configured to:perform a read transaction protocol including: issue read transactionsat any of the nodes; attempt to return rows that are known to bereplicated by at least a quorum of the nodes; and for timing out of theattempting to return rows known to be replicated by at least the quorumof nodes based on the special-node being non-responsive: select a newspecial-node; and repeat attempting to return rows that are known to bereplicated by at least the quorum of the nodes.
 22. The apparatus ofclaim 20, wherein: upon determining a client device timed out duringwaiting for receiving an acknowledgment from the multi-master database,the client device obtains status of the write transaction by issuing atleast one read transaction; selection of a special node comprises anelection; and the special node is any of the nodes.
 23. The apparatus ofclaim 22, wherein: the election is performed by the processor using aconsensus protocol; and upon determining timeout of the at least oneread transaction, repeating, by the client device, the at least one readtransaction.
 24. The apparatus of claim 21, wherein: selection of thenew special-node is performed by the processor upon a transactionfailure or non-responsiveness of the special node; and the quorumfurther includes a number of other nodes selected based on a durabilityrequirement.
 25. The apparatus of claim 21, wherein the processor isfurther configured to: assign weights to the nodes including the specialnode, wherein a weight assigned to the special-node exceeds weightsassigned to remaining nodes, and quorum voting is based on a sum ofvoting nodes weights being equal to or greater than a quorum threshold.